Conventional electrical power systems utilize a synchronous generator for generating AC power. Particularly, such a generator includes a rotor and a stator having a stator coil. In applications such as an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly variable. This variable frequency power is converted to constant frequency power using a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 volt AC power at 400 hz. Such known converters are controlled by a generator/converter control unit (GCCU).
In order to provide aircraft engine starting, such known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control to the generator to energize the stator coil and thus develop motor power to start the engine. The components required in such a start control increase the weight of the aircraft and take up valuable space. To minimize the size and weight of such start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU for the start control.
The present invention relates to a control for a synchronous motor which uses a pulse width modulation (PWM) controlled inverter. One example of such a control is described in Roe et al. U.S. patent application Ser. No. 285,117, filed Dec. 16, 1988, entitled "Control for Producing a Low Magnitude Voltage at the Output of a PWM Inverter", and assigned to the assignee of the present invention. Particularly, such control includes a resolver which develops a signal representing the position of a rotor. A wave form generator is responsive to the rotor position signal and comprises a lookup table to generate three phase displaced half cycle reference wave forms. Such wave forms are commonly referred to as six-step wave forms. These reference wave forms are combined with a duty cycle wave form and a modulation wave form to derive first, second and third pulse width modulated switch control signals for driving the switches in a three phase bridge inverter circuit.
Such a control for a PWM inverter may be utilized in the start mode of operation. Nevertheless, it is desirable to simplify the overall control system.